Some current desktop personal computers include respective television tuner cards. Typically, such a tuner card is connected via a coaxial cable to an antenna for receiving terrestrial broadcast signals and an antenna dish for receiving satellite broadcast signals, both of which are mounted on a rooftop.
For these several years, mobile apparatuses, such as notebook personal computers (PCs) and personal digital assistants (PDAs), having the capabilities of performing wireless communications via respective small size antennas coupled to the apparatuses, such as the wireless LAN communications, the Bluetooth standard short distance wireless communications and the mobile telephone communications, have been spreading widely. Such mobile apparatuses, however, cannot receive the current terrestrial analog television signals in good condition with respective small antennas coupled to the apparatuses. Thus most mobile apparatuses do not include television tuner cards. In the future, however, when terrestrial digital television broadcasting becomes available in Japan, such mobile apparatuses will be able to receive, with respective small antennas, relatively high quality RF signals with carriers modulated with digital image signals. Thus it is expected that, in the future, many mobile apparatuses will have a capability of receiving terrestrial digital television broadcast signals.
In general, however, RF signals of a plurality of terrestrial television broadcasts exhibit lower frequencies and a wider frequency range than those of the wireless LAN communications and other short distance wireless communications. Transmission and reception of RF signals of a plurality of terrestrial television broadcasts require a relatively large size antenna, which cannot be accommodated in a mobile apparatus.
Japanese Patent Application Laid-Open Publication (JP-A) No. HEI 11-251825 discloses a small inverted-F antenna which can be accommodated in a small mobile communication apparatus. This inverted-F antenna has an antenna length equal to one quarter of the wavelength λ of the transmitted and received RF signal, and is suitable for implementing in a mobile communication apparatus. The inverted-F antenna is associated with a resonant circuit for controlling radiator length in accordance with the frequency and also a resonant circuit for controlling the distance between the feed point and the ground in accordance with the frequency.
Typically, in the inverted-F antenna, a variable capacitance diode is employed for a resonant circuit to control its resonance frequency. However, the typical range of tuning capacitance of such a variable capacitor is relatively small. Widening the range of variable capacitance requires an undesirable relatively high control voltage to be applied to the capacitor.
Japanese Patent Application Laid-Open Publication (JP-A) No. HEI 10-284919 discloses a small antenna apparatus for a mobile communication unit for transmitting and receiving a wide range of RF signals. This antenna apparatus includes the antenna proper and a frequency tuning circuit. The tuning circuit includes an arrangement formed by a diode and a capacitor coupled in parallel. The resonance frequency of the antenna is varied by controlling the on/off operation of the diode.
The inventors have recognized the need for an inverted-F antenna, which has a variable capacitor controllable with a relatively low control voltage, and which has a wider frequency range for receiving RF signals of a plurality of terrestrial broadcasts.
An object of the present invention is to provide an inverted-F antenna that has a wider frequency range.
Another object of the invention is to provide tuning of a resonance frequency of an inverted-F antenna with a wider frequency range.